[mod_python] cookies/session management

I am currently working on session management for my project.  One thing
I can't
seem to do for the life of me is to clear out a cookie on the browser
thats
connecting to the site.

    r.headers_out["set-cookie:"] = \
        "IRWB_SESSIONID=%s;" \
	  "Expires: %s;" \
        "Cache-control: no-cache;" \
        "Max-Age=0" % (sessionID, time.time() - TIME_YEAR)

Is what I use to try and expire a cookie immediately, it may be
overkill, but
still, it doesn't work as I wish.  However, it does work if I close the
browser
and reopen it to the given site.

Am I doing something wrong?  I am pretty sure I could clear out cookies
this
way in Visual Interdev/VB/ASP.

Thanks in advance,
Richard.